It is known to produce mixed pigments of titanium dioxide by a mechanical mixing of titanium dioxide pigments with, e.g., zinc oxide, calcium sulfate or barium sulfate.
Processes are known in which composite titanium pigments are produced and which comprise a precipitation of titanium oxide hydrate in the presence of a white carrier, such as barium sulfate, from a titanium-containing solution of a mineral acid by heating and hydrolysis. The precipitate is separated from the mother liquor, washed and calcined (German Patent Specification No. 570,875).
Titanium dioxide is used in its anatase and rutile modifications as a white pigment, which has found wide industrial application and can be made by a sulfate process or by a chlorine process.
In the sulfate process, ilmenite or titanium-containing slag is solubilized with sulfuric acid to provide a solution of titanyl sulfate and that solution is hydrolyzed to precipitate metatitanic acid.
The precipitate is subsequently washed in a sulfuric acid-containing medium until the heavy metal ions which would adversely affect the optical behavior, i.e. chromophores such as iron, chromium, vanadium and copper, have been substantially removed from the suspension to a substantial extent.
Owing to the process conditions the resulting metatitanic acid suspension usually contains between 5 and 10% free and combined sulfuric acid.
In a subsequent calcining step, which is necessary to provide the particle size that is required to achieve pigment properties, the sulfuric acid is eliminated as SO.sub.3. That elimination of SO.sub.3 during the calcining step requires an expensive purification of the exhaust gas.
If a titanium dioxide is required which has not been calcined, i.e., a product having a large surface area, the adhering sulfuric acid must be neutralized first in a neutralizing step, e.g., with a sodium hydroxide solution or ammonia. Such neutralization has the great disadvantage that the soluble salts formed by the neutralization must subsequently be washed out. The washing-out of such salts is very difficult because the neutralized metatitanic acid suspension has poor filtration properties.